This is Steven Salzberg's blog on genomics, pseudoscience, medical breakthroughs, higher education, and other topics, including skepticism about unscientific medical practices. Here's where I can say what I really think about abuses and distortions of science, wherever I see them.

Everyone knows that being obese is very bad for your health. But how overweight do you have to be before you should worry? A new study covering millions of people attempts to answer this question.

The short answer: being a little bit fat isn't so bad, especially if you're already a senior citizen, but the fatter you are, the shorter your life expectancy. Let's dive into the details.

The new study, published in The Lancet, is a combined evaluation (a meta-analysis) of 239 studies that included over 10 million people from four continents: Asia, Australia, Europe and North America. All the studies followed their subjects for a long time, averaging nearly 14 years of observation. The authors (a large consortium called "The Global BMI Mortality Collaboration") wanted to exclude people who might have already been sick, so their study only looked at people who (a) had never smoked, and (b) who lived at least five years after the study began.

This left them with nearly 4 million people, of whom 385,879 died at some time during the course of the study. From this large data set, the researchers computed the risk of death as a function of body mass index (BMI).

[Aside: BMI is a simple function of your height and weight. For example, someone who stands 5'11" and weights 170 has a BMI of 23.7. A height of 5'6" and weight of 150 gets you a BMI of 24.2. You can calculate your own BMI using this calculator.]

The study divided people into six groups:

underweight, BMI 15–18.5

normal, BMI 18.5–24.9

overweight, BMI 25–29.9

obesity grade 1, BMI 30–34.9

obesity grade 2, BMI 35–39.9

obesity grade 3, BMI 40 or above

The main outcome that they studied was mortality (death) from any cause. Of course, one can argue that this is too simplistic, since if someone dies from, say, an auto accident, it probably wasn't due to their weight. But the results were consistent across all four continents, which argues that the study design was probably good. Here are the main findings for each group:

BMI 15–18.5: 47% increased risk of death

BMI 18.5–24.9: no increase (normal)

BMI 25–29.9: 11% increased risk of death

BMI 30–34.9: 44% increased risk of death

BMI 35–39.9: 92% increased risk of death

BMI 40 or above: 171% increased risk of death

Another way to describe these hazard ratios is this: with a BMI above 40, people are 2.71 times as likely to die during any particular time period as people with a normal BMI.

If these numbers seem scary, keep in mind that this is relative risk, not absolute risk. So an 11% increase in risk might mean that your chance of dying increases from 1% to 1.11%; it certainly doesn't mean you have an 11% risk of dying.

To put some real numbers on this risk, consider this comparison: out of 1,075,894 people with a near-optimal BMI between 22.5–25.0, 98,833 died during the course of the study, or 9.2%. (Remember that these data come from 239 different studies, and the average length of followup is 14 years. So fewer than 1% of this group died per year.) Compare this group to people with obesity grade 1, or BMI from 30–35. That group had 330,840 people, of whom 37,318 died, or 11.3%. After various adjustments, this translates into a 44% relative increase, but the actual mortality rate, per year, was about 0.66% versus 0.81% in the two groups.

One mildly positive note: if you're already 70 or older, having a BMI from 25–30 has almost no effect; in other words, it's okay to be a little plump when you're older.

A word of warning to men: the ill effects of obesity are much stronger in men than in women. The study breaks down those hazard ratios by sex, and in each range the risk is higher for men than for women. So for example, if you're a woman with a BMI of 30–35, your hazard ratio is 1.37 (37% higher risk of death), but for men it is 1.70 (70% increase).

There are many, many more details in the study, including a breakdown of how BMI is associated with four major causes of death (heart disease, cancer, respiratory disease, and stroke), and if you're interested in those, you should read the study.

Of course, one can think of many caveats to these findings: people die from all sorts of illnesses, and many of them are not caused by being overweight. For any individual case, we might not be able to say whether someone's weight had anything to do with their illness. Nonetheless, this very large study shows clearly that the more obese you are, the greater your risk of dying. That's precisely what we would expect if obesity was causally linked to mortality.

So is it okay to be a little bit fat? The answer is probably yes: people with a BMI of 25 might view themselves as "a bit" fat, even though they are not overweight. But very high BMIs (and very low BMI, below 18.5) are definitely unhealthy. Unfortunately, no one has an easy answer to the problem of losing weight (despite what you might have heard from Dr. Oz), but if you do have a dangerously high BMI, reducing it will likely be good for your health.

Over the past two decades, anti-bacterial soaps have proliferated. It got to the point, a couple of years ago, where it was difficult to find any hand-washing products in some stores that were not labeled "antibacterial." All the way back in 2001, a study by Eli Perencevich and colleagues found that 76% of liquid soaps on the market contained some kind of antibacterial agent. It only got worse after that.

The primary active ingredients in most of these soaps are triclosan and triclocarban, chemical agents that do indeed kill bacteria. However, as Perencevich pointed out,

"No scientific data have been published to suggest that the use of antibacterial agents in household products prevents infection."

It didn't take long for triclosan to start showing up in freshwater streams and elsewhere in the environment. Triclosan has been detected in the water supply in the U.S., multiple countries in Europe, Canada, Australia, Japan, and Hong Kong. As use of antibacterial soaps proliferated, bacteria resistant to triclosan started to appear–an outcome that any biologist could have predicted, and that many did.

After many years, the FDA finally announced, in December 2013, that manufacturers needed to produce actual data showing that their products worked. They didn't, for the simple reason that antibacterial soaps don't work any better than plain old soap and water.

Finally, this past Friday, the FDA stepped in. They are banning 19 different chemicals, including triclosan. Their announcement states:

"Companies will no longer be able to market antibacterial washes with these ingredients because manufacturers did not demonstrate that the ingredients are both safe for long-term daily use and more effective than plain soap and water in preventing illness and the spread of certain infections."

This has been a very long time coming.

Antibacterial soaps are no more effective than simple hand-washing with regular soap, as Forbes contributor David DiSalvo explained last year. But people have been buying these products anyway, often not realizing that they might be hurting themselves and the environment. As Janet Woodcock, M.D., director of the FDA’s Center for Drug Evaluation and Research, explained in the FDA announcement:

“Consumers may think antibacterial washes are more effective at preventing the spread of germs, but we have no scientific evidence that they are any better than plain soap and water. In fact, some data suggests that antibacterial ingredients may do more harm than good over the long-term.”

Sometimes government regulation is the only effective way to fix a public health problem. Kudos to the FDA for stepping in to protect all of us from potentially deadly outbreaks of antibiotic resistant bacteria. Let's hope that other countries follow the U.S. example.